Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions

Reactions of nucleophiles with 5-(alkoxy)thianthreniumions

Bo Liu, Henry J. Shine* and Wenyi Zhao

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA

Received 3 May 1999; revised 7 July 1999; accepted 9 July 1999

ABSTRACT: Reactions of 5-(alkoxy)thianthrenium perchlorates (1) with weakly basic nucleophiles Brÿ, Iÿ andPhSÿ (Xÿ) in MeCN and DMSO led toSN2 substitution,E2Celimination, and reaction at sulfonium sulfur to extentsdepending on the structure of the alkoxy group (RO) in1 and the nucleophile. Three types of reaction occurred withR = cyclopentyl (1a), cyclohexyl (1b), cis- (1c) and trans- 4-methylcyclohexyl (1d) and cycloheptyl (1e), andXÿ = Brÿ and Iÿ. That is,SN2 reaction gave RX and thianthrene 5-oxide (ThO),E2Creaction gave cycloalkene andThO and reaction at sulfonium sulfur gave X2, thianthrene (Th) and cycloalkanol (ROH). Earlier work with R = Me(1f) and Et (1g) and Xÿ = Iÿ, Brÿ had shown that onlySN2 reaction occurred. In contrast with reactions of halide ions,reactions of PhSÿwith 1b–goccurred only at sulfonium sulfur, giving Th, ROH and PhSSPh (DPDS). For comparisonwith 1, reactions of Ph2S

�OMe (2) with Iÿ and PhSÿ were carried out. Reaction with Iÿ gave only Ph2S=O and MeI(SN2). Reaction with PhSÿ gave very little PhSMe (SN2) but mainly Ph2S, MeOH, and DPDS from reaction atsulfonium sulfur. The differences in nucleophilic pathways (PhSÿ vs Brÿ and Iÿ) in reactions with1 and 2 areattributed to differences in thiophilicities of the nucleophiles. The thiophilicity of PhSÿ dominates its reactions with1and 2. The direction toward products (Th, ROH and DPDS) in these reactions is compounded by the ease ofdisplacement of alkoxide from1 and 2 by PhSÿ, and the ease with which, subsequently, thiophilic PhSÿ attackssulfenyl sulfur in the resulting phenylthiosulfonium ion. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: alkoxysulfonium ions; nucleophilic reactions; thiophilic reactions

INTRODUCTION

Alkoxysulfonium salts have been known for about 60years, since the first one was prepared by Meerwein.1–3

Meerwein’s method, the alkylation of a sulfoxide, is infact the one frequently used for preparing methoxy- andethoxysulfonium salts, utilizing commonly availabletrimethyl- and triethyloxonium salts as alkylatingagents.4 Recently, we found that thianthrene cationradical perchlorate (Th��ClO4

ÿ) reacts easily with avariety of primary and secondary alcohols (Scheme 1),permitting the isolation of crystalline 5-(alkoxy)thian-threnium perchlorates (1).5–7 The availability of thesesalts allowed us to begin a systematic study of theirreactions with nucleophiles. We found, for example, withthe use of 18O labeling, that water reacted with 5-(cyclohexyloxy)thianthrenium perchlorate at sulfoniumsulfur, producing thianthrene 5-oxide (ThO) and cyclo-hexanol.5 Reactions of1 containing acyclic primary andsecondary alkyl groups (R) with bromide and iodide ionswere different, however. The major products were alkylhalide and ThO [Scheme 2, Eqn. (1)]. The products and

the rates of reaction (with primary and isopropyl R)indicated that these reactions wereSN2 displacements.Small amounts of two side reactions were also observed.One [Eqn. (2)] was an elimination that gave alkene andThO, and the other [Eqn. (3)] was reaction at sulfoniumsulfur that gave thianthrene (Th), halogen and the alcohol(ROH).7

We have continued with studies of reactions ofnucleophiles with1 and report here the reactions of1a–e with Brÿ and Iÿ (Scheme 2) in which R is cyclopentyl(1a), cyclohexyl (1b), cis- (1c) and trans-4-methylcy-clohexyl (1d) and cycloheptyl (1e). As shown in Scheme2, these reactions encompassed substitution [Eqn. (1)],elimination [Eqn. (2)] and halogen formation [Eqn. (3)].We also studied reactions of1b–ewith thiophenoxide ion(PhSÿ). In contrast with halide ion reactions, those withPhSÿ occurred only at sulfonium sulfur, with theformation of Th, cycloakanol (ROH) and diphenyldisulfide (X2 = DPDS). For comparison with reactionsof 1 having cycloalkyl R, we studied also reactions of 5-(methoxy)- (1f) and 5-(ethoxy)thianthrenium perchlorate(1g) with PhSÿ. For further comparison with reactions of1, we studied reactions of methoxydiphenylsulfoniumtetrafluoroborate (2) with Iÿ and PhSÿ.

Studies of reactions of alkoxysulfonium ions withnucleophiles are by no means new. The largest systematic

JOURNAL OF PHYSICAL ORGANIC CHEMISTRYJ. Phys. Org. Chem.12, 827–836 (1999)

Copyright 1999 John Wiley & Sons, Ltd. CCC 0894–3230/99/110827–10 $17.50

*Correspondence to:H. J. Shine, Department of Chemistry andBiochemistry, Texas Tech University, Lubbock, Texas 79409, USA.Contract/grant sponsor:Robert A. Welch Foundation;Contract/grantnumber:D-0028.

studieshavebeenin reactionsof hydroxideion, alkoxideions and alcoholswith methoxy- and ethoxysulfoniumions (sincethosestudieswereusedin characterizingtheexchangeof alkoxy ligands at the sulfur atom), theracemizationof sulfoxidesandtheoxidationof alcoholsto aldehydesandketones.2,4,8–12As far aswe areaware,systematicor deliberatestudiesof reactionsof alkoxy-sulfoniumions with halide ions havenot beenreported.Reactionsof chloride and bromide ion with particularalkoxysulfonium ions, coincidental,for example,with,andarisingfrom studiesof, oxidationsof alcohols,havebeenreported,13–16 and we shall refer further to thesereactions.We shallreferfurther,also,to thereductionofsulfoxidesby HI17 and the role of halide ions in theracemizationof sulfoxides,18 and in the exchangeofethoxidegroupsin anethoxysulfoniumion.19 Reportsofreactionsof alkoxysulfoniumions with thiolate ions arealso not numerous.Oae and Kim20 have reportedthereaction of ethoxydiphenylsulfonium tetrafluoroboratewith p-tolyl thiolateandKobayashiandco-workers21,22

havereportedreactionsof arylmethylmethoxysulfonium

saltswith PhSÿ. We shall refer to thesereactionsalso.We have been unable to find any other reports ofreactionsof thiolateionswith alkoxysulfoniumsalts.

RESULTS AND DISCUSSION

Reactions with Iÿ and Brÿ

Reactionswere carriedout in dry acetonitrile(MeCN).Thesourceof Iÿ wasKI. Two sourcesof Brÿ wereused.BecauseKBr is not sufficientlysolublein MeCN, it wasused,asoneof thesources,in thepresenceof 18-crown-6(18C6). A second source was tetrabutylammoniumbromide(TBAB). Reactionproducts,exceptI2 andBr2,were assayedwith quantitative gas chromatographic(GC) analysis.Becausesalts such as 1a–g decomposewheninjectedinto a hot gaschromatographinlet,5 it wasnecessaryto ensurethatreactionwascompletebeforethefinal assaywasmade.To do that, assaysof the reactionmixturesolutionweremadeattimedintervalsuntil, whenreaction appearedto be complete, completion wasverified by injecting a small amountof aqueousK2CO3

into the reaction solution. Salts such as 1a–g areconverted into ThO and ROH by aqueousK2CO3.Therefore,completionof reactionwith halide ion wasindicatedby invariantassaybeforeandafter injectionofaqueousK2CO3. In Tables1 (for Iÿ) and2 (for Brÿ) onlythe final assayof the dry reactionsolution is reported.Iodine wasassayedby titration of an aliquot of solutionafteradditionof aqueousK2CO3. Assayof Br2 couldnotbe made directly, and therefore it was assayedafterconversioninto I2 by additionof KI.

Reactions with Iÿ. Six productsare listed in Table 1:cycloalkene (ene), cycloalkyl iodide (RI), ThO, Th,cycloalkanol(ROH)andI2. Theamountof enewassmall(6.6%) and the amountof RI large (89%) from 1e. Incontrast,theamountof enewaslarge(about50%)andRIsmall (2–6%)from 1b andc. From 1a, 21% of eneand55%of RI wereobtained,whereasfrom 1d, theamountswere approximately35 and 38%. It is apparentthat 1e

Scheme 1

Scheme 2

Copyright 1999JohnWiley & Sons,Ltd. J. Phys.Org. Chem.12, 827–836(1999)

828 B. LIU, H. J. SHINE AND W. ZHAO

behavedmuchlike a primarysalt7 in undergoingmainlySN2 displacement,whereas1a andd to someextent,and1b andc to major extents,succumbedto elimination.

Theduality of pathways,substitutionandelimination,reportedin Table1 (andTable2),andoccuringto amuchlesserextent in our earlier work, parallels competingsubstitution and elimination that was uncovered inreactionsof alkyl halides and tosylateswith weaklybasicnucleophilesabout40 yearsago.The eliminationreactionis a lesserknownbutwell recognizedpartof thespectrumof E2 eliminations. When first reported byWinstein et al.23 with cyclohexyl substrates,it wascalled, for want of a betterunderstanding,the ‘merged’mechanism,connotingan elimination that occurredin apathway destined for SN2 substitution. Thus, thiseliminationaccompaniedSN2 reactionsof weaklybasic,good nucleophiles,and gave rise to the classification,albeit somewhatcontroversially,E2C elimination.24–26

This is theeliminationthat iodideion causesin reactionswith 1a–e, muchmorepronouncedthanwasobtainedinour earlier work with primary and acyclic secondaryalkyloxy groups.7 The data in Table 1 show that E2Creactionbecomesmore competitivewith SN2 on goingfrom cycloheptyl to cyclopentyland, lastly, cyclohexylalkyl groups.Reactionswith the cyclohexylcompounds1b–d areparticularlystriking in that theyparallelresultsreportedyearsagofor cyclohexylhalidesandtosylates.That is, Winstein et al.23 noted that cis-4-tert-butylcy-clohexyl tosylategavemoreenethanthe trans isomerinreactionswith Iÿ, Brÿ andPhSÿ. Eliel andHaber27 foundin reactionswith PhSÿ that the rateof elimination fromcis-4-tert-butylcyclohexyl bromide was 66 times fasterthan from the trans isomer.In thosecis substrates,theleaving group is axially oriented. That is also theorientationof the thianthreniumoxygroupin the ring of1c,6 andTable1 showsthatmoreene(52%) is obtainedfrom the 1c thanfrom its trans isomer,1d (35%).Thus,1c and d have the characteristicsof E2C reactionsobservedearlier with much simpler substrates.It isnoticeablethat the amountsof eneand RI formed also

from the cyclohexyloxy salt (1b) are similar to thoseformedfrom 1c ratherthan1d, which hasan equatorialthianthreniumoxygroup.At first sight,thiswaspuzzling,sincethe dominantorientationof the thianthreniumoxygroupin cyclohexyl ring of 1b is equatorial.6 However,1b, butnot1candd, undergoesring inversionin solution,and at room temperatureapproximately30% has theaxially oriented thianthreniumoxygroup.6 We deduce,therefore,that the axial conformer reacts in the E2Cmode more rapidly than the equatorialconformer, asfound by earlier workers with simpler systems,thusleading1b into a patternof reactioncomparableto thatfor 1c. It is apparentthat, in SN2/E2C terminology,ThOis a good leaving group, and that the cyclopentyl andcycloheptylrings of 1a ande allow moreeasilyfor SN2displacementthanthecyclohexylsystems1b–d. Wehavenotbeenableto find studiesof or referencesto E2C-typereactionswith cyclopentyl and cycloheptyl derivativescomparableto thoseof cyclohexylhalidesandtosylates.

Reactionsthat give RI and enemust also give ThO[Eqns(1) and(2)]. Therefore,thesumof theyieldsof RIand ene should equal the yield of ThO. That this isreasonablycorrect in our resultsis shownby the ratio(RI� ene)/ThOin Table1.

Earlier,we notedthatalthoughwe havenot beenableto find systematicstudiesof reactionsof halideionswithalkoxysulfonium salts in the literature, reactions ofchloride and bromide ions with particular alkoxysulfo-niumionshadbeenreportedbyCoreyandco-workers.13–16

The reactions originated in studies of oxidation ofprimary and secondaryalcoholsthroughthe agencyofanalkoxydimethylsulfonium ion (5) formed in situ froma chlorosulfoniumchloride(3) and/orsuccinimidosulfo-niumchlorideandbromide(4, Scheme3). Coreyandco-workers noted that in some cases oxidation wasaccompaniedby or supersededby halogenation,withthe formation of R1R2CHX and Me2SO. For example,when the succinimidochloride 4 was used,benzhydroland2-cyclohexenolgavethe correspondingalkyl chlor-ides ratherthan the ketones.13 Reactionat ÿ25°C with

Table 1. Products of reactions of 1a±e with KI

Product(%)

Compound ene RI ThO Th ROH I2a Ratiob

1a 21 55 78 19 c 17 0.971b 53 6.0 55 44 43 43 1.071cd 52 2.5e 52 48 46 47 1.051df 35 38 76 23 21 23 0.961e 6.6 89 95 5.4 tr 5.3 1.01

a Yield (%) basedon startingamountof 1.b Sumof (ene� RI)/ThO.c Peakoverlappedwith solvent.d Averageof two experiments.Errorsin tabulatedaverageswere2.0 (ene),0.6 (RI) and1.0 (all otherproducts.e Assaybasedon responsefactor for cis-(4-methyl)iodocyclohexanebecausepuretrans isomercould not beprepared.f Averageof two experiments.Errorsin tabulatedaverageswere1.0 (eneandI2), 2.0 (RI andTh), 3.0 (ThO) and4.0 (ROH).


REACTIONSOF NUCLEOPHILESWITH 5-(ALKOXY)THIANTHRENIUM IONS 829

benzhydrol,2-cyclohexenoland benzyl alcohol in theabsenceof triethylaminegave,in fact, >95% yields ofalkyl chlorides.14 Analogously,geraniolwas convertedinto geranyl bromide with the use of N-bromosuccini-mide. Corey and co-workers attributed alkyl halideformation to cases in which the sulfoxonium ionintermediate(5, Scheme3) is relatively unstable14 andcontains alcohols which correspond with stabilizedcarbocations.13,15

An analogouschlorination has been reported byJohnsonandJones29 in theformationof alkoxysulfoniumions by the oxidation of sulfides with tert-butyl- andisopropyl hypochlorites at ÿ78°C. The initial tert-butoxy-andisopropyloxysulfoniumchlorideswerefoundto be too unstableto be isolated,but could be stabilizedinto isolable hexachloroantimonates by the addition ofantimonychloride. Without this stabilizationthe initialsalts decomposedinto sulfoxide and tert-butyl- andisopropyl chloride; quantitative data for chloroalkaneformationwerenotprovided.Obviously,theformationoftert-butyl chloridecouldnotbefrom anSN2 displacementand, by analogy,isopropyl chloride was not formed inthat way either.The mechanismof chloroalkaneforma-tion in thesecasesremainsunidentified.

It is evident that our reactionsof 1 with iodide (andbromide)ion differ from the halogenationsaccompany-ing theseoxidationreactions.That is, our first reportofalkyl halideformationwaswith examplesof 1 thatwerenot unstablein solutionandthat containedprimary andacyclic secondaryalkyl groups unlikely to representstabilizedcarbocations.In thosereactionsthe ratesofformation of primary alkyl halides clearly fitted SN2characteristics.Furthermore,5-(neopentyloxy)thianthre-nium perchloratefailed to give neopentyl-or any otheralkyl iodide, a finding that was consistentwith SN2characteristicsof alkyl iodide formation. In our presentwork, the behaviorof the cyclohexyloxysalts1b–d fitclearlyinto theSN2/E2C dualityof earlierreports.In onlyonerespectareCoreyandco-workers’resultsanalogousto ours,andthatis thatsaturatedalcohols,ordinarily inertto conversioninto halidesin competitionwith oxidation,could be convertedduring lengthyreactiontimes in theabsenceof triethylamine. Thus, cyclohexylmethanolgave 43% of the chloride after 100h at 5°C, whilecyclohexylmethanoland 2-phenylethanolgave 70% ofthe bromides after 36h at 20°C in reaction withderivative4.14 Thesereactionswould appearto be SN2displacementsanalogousto ours.Coreyandco-workerscommentedthat the useof reagentsderivedfrom othersulfides may extend the scope of their halogenationreaction. Our use of the isolable alkoxythianthreniumsaltsfulfils thatprognosis,at leastinsofarasprimaryandacyclicsecondaryalkyl groupsareconcerned.Suitedalsoto halideformationis themethoxydiphenylsulfoniumion(2), whichasthetetrafluoroborategavemethyliodideanddiphenylsulfoxidequantitativelyin reactionwith iodideion (seeExperimentalsection).

Table 1 lists three more products,Th, ROH and I2.Thesecomefrom athird typeof reactionof compounds1with halide ion, namely reaction at the sulfur atom(Scheme4). This reactionwasobservedto asmallextentin our earlierwork,7 but is seenhereto be significantinreactionsof thecycloalkoxyderivatives.It occursto thegreatestextentwith 1b andc, in which, seemingly,SN2displacementis the most difficult. Thus, reaction atsulfur, as with elimination, competeswith the SN2

Scheme 3

Scheme 4



reaction.Reactionat sulfurshouldgiveequalamountsofTh,ROHandI2, andthis is borneoutby thedatain Table1.

Reactions with Brÿ. Thesereactionswerewith 1a,b ande, and the sourceof bromide ion was either potassiumbromidein the presenceof 18-crown-6or tetrabutylam-moniumbromide.In analogywith iodide ion reactions,threepathwayswereobserved(Scheme2). Thedominantpathwaywith the cyclopentyloxy(1a) and cyclohepty-loxy (1e) derivativeswas bromoalkaneformation.Thiswas the minor pathwaywith the cyclohexyloxyderiva-tive (1b). Instead,eliminationandreactionat sulfoniumsulfur werethemajor fatesof 1b. A complicationin thereactionof 1b wastheadditionof Br2, formedasin Eqn.(3), to cyclohexeneformed as in Eqn. (2) (Scheme2),giving trans-1,2-dibromocyclohexane, designatedRBr2in Table 2. It is possible that small amounts ofdibromocycloalkane were formed also from 1a and ebut we did not searchfor thoseproducts.In terms ofstoichiometry,therefore,the sum of the yields of ene,RBr andRBr2 shouldequalthe yield of ThO. We haveexpressedthat equality as a ratio in Table 2. Also, theyieldsof Th andROH shouldeachequalthe sumof theyields of Br2 plus RBr2. This is also shown to beapproximatelythecasein Table2.

The summary,then,of our reactionsof 1 with Iÿ andBrÿ andof methoxydiphenylsulfoniumion with Iÿ is thatSN2 displacementof an alkyl group prevails untilinhibited by structural features of the alkyl group,whereuponE2C reactionand reactionat sulfur becomesignificant.Reactionat sulfur is shownin Scheme4 toleadto asulfurane(6). Thesulfuraneitself mayreactwithhalide ion, or as proposedby othersin other reactions,17a,bmay lead to a halosulfoniumion (7), from whichproductsareformed.Thelossof analkoxygroupfrom analkoxysulfonium ion in reactionwith a nucleophilehasbeen describedas facile in even the early work ofMeerwein.30 We are unableto distinguishbetweenthetwo pathways.

Reactions of halide ions at the sulfur atom ofoxysulfonium ions, with concomitant formation of asulfurane intermediate, have been proposed earlier.Examples include the racemizationof sulfoxides byHCl18a and the exchangeof a sulfoxide’s oxygenatomwith water,catalyzedby HCl.18b Landini and co-work-ers,17a,binterestingly,foundthat iodide ion in perchloricacidsolutioncausedthereductionof sulfoxides,whereasbromide and chloride ions under the same conditioncausedonly racemization.In all of thesecases,reactionwasinitiatedby theattackof halideion at thesulfuratomof the protonatedsulfoxide, that attack being, in fact,identified as the rate-determiningstep.17a,b Our resultsdiffer alittle from thoseof Landiniandco-workersin thatboth iodideandbromideion causedto someextentwhatis in effect the reductionof compounds1, whereasonlyiodide caused the acid-catalyzedreduction of sulf-oxides.17a The sameworkersreportedan order in thio-philicities of the halide ions, Clÿ, Brÿ, Iÿ as 1:3:87.Similar ordering in the thiophilicities of halide ionshavebeenlisted by Kice31 for catalysesof reactionsofnucleophilesat sulfenyl and sulfynyl sulfur atoms. Inanalogywith suchcaseswemightexpectin our reactionsthat iodide would give muchmoreTh andhalogenthanbromideion in reactionswith 1. Thatexpectationis seenonly in reactionsof 1a, however. It may be that theexpectedorder in thiophilicities is compromisedby thecomplexityof our competingreactionsystems.

Reactions with PhSÿ

Reactionswerecarriedout in MeCN andDMSO over aperiod of 2 h. Completion of reaction was verified insomecasesby following separatelythedisappearanceofthe sulfonium ion with NMR spectroscopy.A smallexcess of sodium thiophenoxide (PhSNa) over theamountof sulfoniumsalt(1 and2) wasused,correspond-ing to thestoichiometryof Schemes5 and6. Thiophenol(PhSH), approximately equivalent to the amount of

Table 2. Products of reactions of 1a, b and e with bromide ion

Product(%)

Compound Sourcea of Brÿ ene RBr RBr2 ThO Th ROH Br2 Ratiob

1a TBAB tr 89 —c 94 5.1 —d 6.2 0.951a KBr–18C 1.5 89 —c 100 3.5 —d 2.0 0.911be KBr–18C 3.9 4.2 43 55 45 45 8.2 0.931ef TBAB 1.0 89 —c 93 8.9 1.3 9.6 0.971eg KBr–18C 0 97 —c 97 5.1 3.7 —c 1.00

a TBAB is tetrabutylammoniumbromide;KBr–18C is potassiumbromide� 18ÿcrown-6.b Sumof (ene� RBr� RBr2)/ThO.c Not soughtor measured.d Peakoverlappedwith solvent.e Averageof two experiments.Errorsin tabulatedaverageswere2.4 (ene),0.6 (RBr), 1.0 (RBr2 andThO), 2.0 (Th), 3.0 (ROH) and4.8 (Br2).f Averageof two experiments.Errorsin tabulatedaverageswere1.0 (ene),5.0 (RBr), 3.0 (ThO), 4.1 (Th), 1.3 (ROH) and3.4 (Br2).g Averageof two experiments.Errorsin tabulatedaverageswere1.0 (RBr), 2.0 (ThO), 0.3 (Th) and1.8 (ROH).



PhSNa,wasincludedin all runs.UnlessPhSHwasused,largeamountsof sulfoxide(ThO from 1, Ph2SOfrom 2)wereformed.(InsofarasusingaddedPhSHis concerned,we found that this wasthe practicein oneof the earlierstudies,27 but not in others. Where E2C occurred,ofcourse,thiol wasformedin situ, aspointedoutby Cooketal.32 with p-nitrothiophenoxideand by Bordwell andMrozack33 with p-methoxythiophenoxide. Routineuseofanexcessof PhSHwasnotedby Arnoneetal.34 in studiesof SRN1 reactions.)In thepresenceof PhSHtheproductsof reactionwere substantiallyonly alkanol (ROH), theparentsulfide (Th from 1, Ph2S from 2) and diphenyldisulfide(DPDS).Noneof the SN2 product(PhSR)wasobtainedfrom reactionsof 1 and only small amountsfrom reactionsof 2. PhSHitself did not reactwith thealkoxysulfonium salts.Theresultsarelistedin Tables3–5. In theseTables,the yieldsof products,exceptDPDS,are from GC analysesof reactionsolutions.Becauseanexcessof PhSNaandPhSHwasused,theyieldsof DPDSwere assayedby GC but only after PhSH and unused

PhSNahad beenremovedas describedin the Experi-mentalsection.We wereableto assaywithout difficultytheformationof cycloalkanolsfrom reactionsof 1b–einMeCN,andof MeOHandEtOHfrom reactionsof 1f andg and2 in DMSO.Directassayof MeOHandEtOHfromreactionsof 1f andg and2 in MeCN werenot possible,apparentlybecauseof the insolubility of NaOMe andNaOEt in MeCN. Thesealkoxideswere not convertedinto the alcohols in these circumstances.Conversionresistedtheadditionof a largeexcessof PhSH,andwasachievedonly with the addition of a small amountofwater (0.2ml) to the reaction solution (10ml) afterreactionwascomplete.Assayof MeOH by GC wasthensuccessfuland is reportedin Tables4 and 5. AssayofEtOH wasnot possibleevenwith this treatmentbecausethe GC peak overlappedthat of MeCN. Somecontrolexperimentsonthefateof sodiumalkoxidesin MeCNareincludedin the Experimentalsection.We concludethatthe formationof cycloalkanolfrom 1b–e in MeCN wasdriven to completionby reactionof PhSHwith soluble

Scheme 5

Scheme 6

Table 3. Products of reaction of 1b±e with PhSNa

Product(%)a

Run Compound ROHb Thb DPDSb,c ThOd

1 1b 99 94 105 5.12 1c 101 94 100 7.73 1d 96 96 103 3.74 1e 96 97 99 6.2

a Average of two experimentswith 1b–d and three with 1e. Allexperimentswerein MeCN at roomtemperature.b Errorsin tabulatedaverageswere1–5.c After separationby columnchromatography;GConcolumnA. Yieldis basedon thestoichiometryof Scheme5.d Errorsin tabulatedaverageswere3.3 (1b), 4.1 (1c), 1.6 (1d) and1.0(1e).

Table 4. Products of reaction of 5-methoxy- (1f) and 5-ethoxythianthrenium perchlorate (1g) with PhSNa

Product(%)a

Run Compound Solvent ROHb Th DPDSd ThO

1 1f DMSO 98 99 104 1.72 1f MeCN 105 99 105 2.53 1g DMSO 96 99 102 1.04 1g MeCN c 95 103 4.2

a Averageof threeassaysin eachrun.Theaverageerrorsfor assaysofROH, Th and ThO were: (run 1) 0.05,1.7, 0.18; (run 2) 0.93,0.30,0.15;(run3) 0.03,1.3,0.22;(run4) –,0.12,0.18.In all experimentsthemolar ratio of PhSNato 1 was ca 2.5, and PhSHequivalentto theamountof PhSNawasalsoused.b On columnE.c EtOH overlappedthe solventin GC traces.d After separationby columnchromatography;GConcolumn A. Yieldis basedon the stoichiometryof Scheme5.

Table 5. Products of reaction of methoxydiphenylsulfoniumtetra¯uoroborate (2) with PhSNa

Product(%)

Run Solvent MeOHa Ph2Sb DPDSc Ph2SOb PhSMeb

1 MeCN 98 98 110 tr 1.62 DMSO 94 89 102 9.7 3.1

a ColumnE wasused.An averageof threeassaysis givenwith errors2.1%(run 1) and1.0%(run 2). MeOH wasfound in run 1 only afteradding 0.2ml of water to the suspensionbefore assay.MeOH wasfoundandassayedin run 2 without additionof water.b ColumnA wasused.An averageof threeassaysis givenwith errorsin yields (%) for Ph2S, Ph2SO, PhSMe,respectively,of 0.80,–,0.03(run 1) and0.40,0.17,–(run2).c After separationby columnchromatograph;GC on columnA. Yieldis basedon the stoichiometryof Scheme6.



alkoxide.Theformationof MeOHandEtOHfrom 1f andg and2 in DMSOwassimilarly drivenor mayhavebeendriven to completionby small amountsof water in theincompletelydriedsolvent.

Reactions with 1b±e

Table3 showsthesubstantiallyquantitativeformationofcycloalkanol,Th andDPDS.Smallamountsof ThOwereobtained.In principle,ThOshouldbeformedfrom eitherSN2 displacementof the cycloalkyl groupor from E2Celimination, but no indicationsof PhSRand ene werefoundin theGC traces.Thesereactionsof 1b–earethusin greatcontrastwith thoseof the halide ions, in whichSN2 and E2C were dominant. The reactionsare alsounlike thoseof cyclohexylhalidesandtosylateswith arylthiolates.For example,Eliel and Haber27 found almostequal amountsof substitution and elimination in thereactionsof PhSÿ with bromocyclohexaneandcis- andtrans-4-(tert-butyl)bromocyclohexane, the ratio of ratesfor the two reaction types being 1.13, 1.04 and 1.17,respectively.Reactionof cis-4-tert-butylcyclohexyl to-sylategave40% of 4-tert-butylcyclohexene.27 The lastreactionin thehandsof Winsteinandco-workers23 gave48% of elimination. McLennan28 obtained 55% ofcyclohexenefrom reactionof PhSÿ with bromocyclo-hexane.We repeatedthat reactionandconfirmedthat inour hands, too, large amounts of cyclohexenewereformed (seeExperimentalsection).Bordwell and Mro-zack33 reportedthat,whereas9-methylfluoreneanionledto >90% substitution,and 6-bromo-2-naphthoxideionled to >90% elimination, in reactionwith bromocyclo-hexane, 4-methoxythiophenoxideion caused equalamountsof substitutionandelimination.Thus,althoughour compounds1b–d can be regardedas cyclohexanescarrying a good leaving group (ThO), they have nopropensityto engagein the SN2 andE2C reactionsthatsuit a good leaving group. The nucleophilic natureofPhSÿ is such that, instead, given the opportunity, itprefersto reactwith 1b–d at sulfoniumsulfur.

Reactions with 1f and g and 2. Preferencefor reactionat sulfoniumsulfur is alsoseenin the reactionsof PhSÿ

with 1f,g (Table4) andwith methoxydiphenylsulfoniumtetrafluoroborate(2, Table5). Wechose1f andg becausewe had found earlier7 that they underwentalmost totalSN2 reactionwith Iÿ and Brÿ. In contrast,eachgave

quantitativeamountsof Th, ROH andDPDSin reactionwith PhSÿ. Noneof the displacementproducts,PhSMeandPhSEt,werefound.Wechoseto work with 2 soastocharacterizethe modeof reactionof simpleralkoxysul-fonium salts with PhSÿ and Iÿ. Reactionwith Iÿ (seeExperimentalsection)gave98–99%of MeI andPh2SO,indicative of SN2 displacementof methyl. Only smallamountsof diphenyl sulfide (Ph2S) were formed, thatwouldbeconsistentwith reactionatsulfur.ReactionwithPhSÿ, however(Scheme6), gavepredominantlyPh2S,MeOH and DPDS. About 9% of ThO was formed inreactionin DMSO, suggestiveof SN2 displacement,butonly 3.1%of PhSMewasfound(Table5).

Ourresultsshowthatin contrastwith halideionsPhSÿ

hasa strongpreferencefor reactionat sulfur in reactionswith 1 and2. OaeandKim20 foundanalogouslythatonlyPh2S (90%) and p-tolyl disulfide (80%) were formedwhenp-tolyl thiolate reactedwith ethoxydiphenylsulfo-niumtetrafluoroboratein anhydrousethanol.Therelativethiophilicity of PhSÿ comparedwith halideionsis all themorenoteworthywhenrelatedto the carbonnucleophi-licity of thesenucleophiles.For example,the ratio ofreactivitiesPhSÿ/Iÿ in methanolsolutiontowardmethyltosylate(MeOTs)is 321andtowardmethyl iodide(MeI)it is 313.35,36 In sulfolane solution, this ratio ofreactivities toward MeI is 37000.37 The ratio fordisplacementof methyl from cobalt-boundmethyl inmethyl cobalt(III)phthalocyanine in dimethylacetamidesolutionis 41000.37 Intrinsically, therefore,especiallyinanalogywith reactionwith MeOTs, one would expectPhSÿ to reactmoreeasilythanIÿ (andBrÿ) at thealkylgroup and displaceThO from 1 and Ph2SO from 2.Reaction at both sulfonium sulfur (43%) and carbon(12%)did ocurin reactionof PhSÿ with methoxymethyl-p-tolylsulfonium tetrafluoroborate (8) in dichloro-methane(Scheme7).21 In that case, methoxide ion,displaced from the methoxysulfonium ion by PhSÿ,reactedfurther with 8 to give dimethyl ether.No suchreactionwasfound in our work.

Thecollectivedatashowthatreactionof PhSÿwith thesulfur atomof an alkoxysulfoniumion is very facile. Interms of Scheme4, the sulfurane(6) and phenylthio-sulfonium ion (7) are formed preferably. Once 7 isformed, furthermore,it must reactrapidly with thiolateion irreversibly.Measurementsof thiophilicity of thiolateions comparableto thoseof halide ions17a,31,38do notseemto have beenmade,althoughKice and Rogers39

found that SCNÿ reactedmore rapidly than Iÿ at thesulfenyl sulfur atom of PhSS�(OH)Ph. Edwards andPearson40 notedthat,amongnucleophiles,RSÿ andPhSÿ

rank high in their ability to breaka sulfur-sulfur bond,while Kice31 deducedthationssuchasR2S

�SPhundergonucleophilicsubstitutionat sulfenyl sulfur with tremen-dousalacrity. More recentreportshavebroughtout thevery facile, SN2-like natureof the attackof thiolateionson thedisulfidebond,41 from which, by comparison,theeven more facile SN2-like displacementof sulfonium

Scheme 7



sulfur (e.g. in 7) by thioloate would be expected.Thesummationof ourwork andtheearlierreports20–22is thatthiolate ions react selectively at the sulfonium sulfuratomof alkoxysulfoniumions,facilitated,perhaps,by thefacile expulsionof thealkoxy group.20 Oncethatoccurs,it is followed by anothervery facile product-formingreactionof thiolateat thesulfenyl sulfur atomof (in ourcases)a phenylthiosulfoniumion.

EXPERIMENTAL

Solventsweredriedasdescribedearlier.6 Cycloheptanol,iodocyclohexane, bromocyclopentane,bromocyclohex-ane, bromocycloheptane and trans-1,2-dibromocyclo-hexane were obtained from Aldrich Chemical andcyclopentanolfrom ArapahoeChemical.Iodocyclopen-tanewasprepared42 by reactionof cyclopentanol(1.0g,12mmol) with iodotrimethylsilane(5 g, 25mmol) in40ml of CH2Cl2 andwaspurified by columnchromato-graphyon60–100-meshsilicagelwith pentaneaseluent,giving 0.62g (3.16mmol, 26%) of product with asatisfactory1H NMR spectrum.Iodocycloheptanewaspreparedsimilarly and was distilled after chromatogra-phy failed to give a satisfactoryproduct, giving 1.4g(6.25mmol, 71%), b.p. 56–57°C (2 mmHg); lit.43 b.p.92°C (14mmHg). cis-(4-Methyl)iodocyclohexanewasprepared from trans-4-methylcyclohexanol (830mg,7.28mmol) andiodotrimethylsilane(2.90g, 14.5mmol)in 40ml of CHCl3. Only a small amountof productwasobtained after 2 days of stirring, and stirring wascontinuedat room temperaturefor 20 days.Iodine wasremovedby washingwith sodiumthiosulfateandwater.Evaporationof the dried solution and distillation gave460mg (2.05mmol, 28%) of product, b.p. 45–46°C(2 mmHg). The product had a satisfactory 1H NMRspectrumcorrespondingto the cis structure,concordantwith themechanismof formation.42 Attemptsto preparetrans-(4-methyl)iodocyclohexane from cis-4-methylcy-clohexanol gave a mixture of cis- and trans-iodides.Physicaldatafor theisomericiodidescouldnot befoundin theliterature.Cyclohexylphenylsulfidewaspreparedby reactionof iodocyclohexanewith PhSNa,and wasseparatedfrom DPDSby chromatographyonacolumnofsilicagelelutedwith light petroleumether,b.p.37–56°C.The product44 had a satisfactory1H NMR spectrum.Methoxydiphenylsulfonium tetrafluorborate(2) waspre-pared as described earlier.4 Sodium thiophenoxide(PhSNa)waspreparedby reactionof thiophenol(PhSH)with dispersedsodium in boiling diethyl ether underargon.The product(95%) wasfiltered andwashedwithdry diethylether.It wascompletelysolublein MeOHandDMSO but only partly solublein MeCN. For example,21mg remainedundissolvedwhen 66mg in 10ml ofMeCNweretreatedwith asonicvibrator.Similarly, 36%remainedundissolvedwhen18-crown-6wasincludedina heatedsolution. The recoveredsolid was itself only

partly soluble in fresh MeCN. We did not proceedfurther. The insolubility of PhSNain MeCN has beennotedearlier.45 ThermogravimetricanalysisshowedthatthePhSNadid notcontainvolatilematerialsuchaswater.

Preparation of 1a±g. The preparationsof 1b–d, f andgfrom reactionsof thianthrenecation radical perchlorate(caution: a warning about explosivenesshas beengiven46) havealreadybeendecribed.6,7 Compounds1a(66%) and 1e (69%) were preparedsimilarly and hadm.p. (decomp.)63–68and70–71°C, respectively.Eachhadthecharacteristicaromatic1H NMR spectrumof a5-(alkoxy)thianthreniumperchlorate.6,7 The multiplet forH-1' of 1aat� 5.12andof 1eat� 4.84were,asexpected,approximately0.8ppmdownfieldfrom thesignalsof thecorrespondingalcohols.

Assays of products. Products of reaction (excepthalogen)of 1a–ewith halide ions wereassayedby GCon a Varian Model 3700 gas chromatographand aSpectra-PhysicsModel 4290 integrator–recorder.Threestainless-steelpackedcolumnswereused,eachof 1/8 ini.d.: A, 10%OV-101on80–100-meshChrom-WHP,4 ft;B, 10% Carbowax20 M on Chrom-WHP,6 ft; C, 10%OV-17 on 80–100-meshChrom Q11, 6 ft. ColumnsAandC wereheldat 50°C for 2 min andrampedto 250°Cat 12°C minÿ1; column B was heatedsimilarly, but toonly 100°C. Assaysof productsof reactionswith PhSNaweremadewith columnA andwith two othercolumns:D, capillarySE-54(SupelcoNo. 2-4001)andE, capillarySPB-20 (Supelco No. 2-4086). Each was 30 m�0.25mm i.d., film thickness0.25mm. D was heatedat50°C for 2 min andrampedat 12°C minÿ1 to 250°C; Ewasheatedsimilarly andrampedto 100°C. In all casesthe injector wasat 250°C anddetectorat 300°C.

Reactions of 1 with KI. An exampleis given with 1a(entry 1, Table 1). A solution of 1a (44.0mg,0.110mmol) and KI (46.9mg, 0.394mmol) in 5 ml ofMeCN containingboth naphthaleneand 2-butanoneasGC standardswasstirredin a septum-cappedvolumetricflask.Aliquots weretakenfor GC analysisafter 35, 140and280min of stirring,afterwhich0.3ml of 2 M K2CO3

was injected. Stirring was continued for 2 h and GCanalysiswasrepeated.Theproducts(mmol� 102) of thethird and fourth (in parentheses)assayswerecyclopen-tene2.30 (2.20), iodocyclopentane 6.00 (6.00),Th 2.10(2.13) and ThO 8.60 (8.80). Cyclopentanolwas notassayedbecauseits GC peakwastoo broadandshallowand overlappedwith the solventpeak.The third assaysarelistedin Table1. An aliquotwastakenafter thethirdassayfor titration of iodine. For this, 0.3ml of K2CO3

was addedto the aliquot and was followed by titrationwith Na2S2O3. ColumnE wasusedfor all productsexceptcyclopentene,for which columnB wasused.

Reactionsof 1b–e were carried out similarly withsmall variationsin concentrationsandsamplingtimes.



Reactions of 1 with TBAB. An exampleis givenwith 1a(entry 1, Table 2). A solution of 1a (41.6mg,0.104mmol) and TBAB (115mg, 0.357mmol) in 5 mlof MeCNcontainingbothnaphthaleneand2-butanoneasGC standardswasstirredin a septum-cappedvolumetricflask.Aliquots weretakenfor GC analysisafter 10, 50,120,285and345min of stirring.Products(mmol� 102)after the third and fifth (in parentheses)assayswerecyclopentene trace(trace), bromocyclopentane 9.33(9.29),Th 0.523(0.530)andThO, 9.92(9.79).The fifthassaysarelisted in Table2. After 6 h of stirring, 500mgof KI wereaddedto thesolutionandtheliberatediodinewastitratedwith Na2S2O3. Theresultis expressedasBr2in Table 2. Column B was usedfor cyclopenteneandcolumnC for all otherproducts.

Reactions of 1 with KBr±18-crown-6. An exampleisgiven with 1b (part of entry 3, Table 2). A solution of427mg(0.103mmol)of 1b in 5 ml of asolutionthatwas0.0717M in KBr and 0.132M in 18-crown-6 andcontaining both naphthaleneand 2-butanoneas GCstandardswas stirred in a septum-cappedvolumetricflask.Aliquotsweretakenfor GCassayafter10,170,235and370min. The fourth assayis listed in Table2. Afterthat assay,500mg of KI wereaddedto thesolutionandiodine was titrated as usual. Column B was used forassaying cyclohexene and column C for all otherproducts.

Reactions of 1 with sodium thiophenoxide in thepresence of thiophenol. An exampleis given with 1b(part of entry 1, Table 3). A solution of 71.0mg(0.171mmol) of 1b and45ml (48.3mg, 0.439mmol) ofPhSH in 10ml of MeCN containing biphenyl as GCstandard was stirred with 57.8mg (0.438mmol) ofPhSNa in a septum-cappedvolumetric flask for 2 h.The solution was assayedtwice on column A givingin mmol (average)0.169� 0.0015 (99%) of cyclohex-anol, 0.157� 0.0001 (92%) of Th, 0.0029� 0.0005(1.7%) of ThO and0.215� 0.002(126%,basedon theamountof 1b) of DPDS.Following the GC assays,thesolution was pouredinto 50ml of 2 M NaOH and theaqueoussolutionwasextractedwith 3� 25ml of diethylether.Thedried (MgSO4) ethersolutionwasevaporatedto drynessandthesolid residuewasplacedon a columnof silicagel from which94.3mgof amixtureof biphenyl,Th and DPDS was eluted with light petroleum.Thismixture wasdissolvedin 5 ml of MeCN andassayedbyGC on columnA to give 0.148mmol (87%) of Th and0.175mmol (102%,basedon1b) of DPDS.ThisassayofDPDSis reportedin Table3.

Similar reactionswith 1c–ewerecarriedout. In somecases,both biphenyl and 2-butanone(for ROH assay)were used as internal GC standards.In these runs,columnsA and D were used.Reactionswith 1f and gwere carried out similarly but in DMSO and MeCN(Table4).

Reaction of methoxydiphenylsulfonium tetra¯uorobo-rate (2) with sodium thiophenoxide in the presence ofthiophenol. An exampleis givenwith entry2,Table5.Asolutionof 71.9mg (0.237mmol) of 2 and60ml (64mg,0.582mmol)of PhSHin 10ml of DMSOcontainingbothbiphenyland2-butanoneasGCstandardswasstirredin aseptum-cappped volumetric flask with 76.8mg(0.582mmol)of PhSNafor 2 h.Thesolutionwasassayedthree times on column A, giving in mmol (average)0.212� 0.020 (89%) of Ph2S, 0.023� 0.002 (9.7%) ofPh2SO,0.357� 0.002(151%,basedon theamountof 2)of DPDS, 0.00745� 0.0009 (3.1%) of PhSMe and0.222� 0.0036 (94%) of MeOH. Following the GCassays,the solutionwaspouredinto 50ml of 2 M KOHandworkedup asdescribedearlier to give 105mg of amixture of biphenyl, Ph2S and DPDS. GC assaygave0.200mmol (84%) of Ph2S and 0.242mmol (102%,basedon theamountof 2) of DPDS.Thisassayof DPDSis reportedin Table5.

Reactions of 1b and f and 2 with sodium thiophen-oxide in the absence of thiophenol. The PhSNawascrystallizedfrom ethanol,washedwith hexaneanddriedundervacuum.A solutionof 34.0mg (0.0821mmol) of1b in 5 ml of MeCN containing naphthaleneas GCstandardwas stirred with 35.6mg (0.0270mmol) ofPhSNafor 2 h.GCassaygave0.0524mmol (64%)of Th,0.0311mmol (38%) of ThO, 0.0771mmol (94%) ofcyclohexanoland 0.0521mmol (63%) of DPDS. Ananalogousreactionwith 1f gave55%of Th, 43%of ThOand62%0f DPDS.The GC peakfor MeOH overlappedthatfor thesolvent(columnA). Similarly, 2 gave46%ofPh2S, 54%of Ph2SO,71%of DPDSand53%of MeOH(columnE). Thesereactionsmaybecomparedwith thosein thepresenceof PhSH(Tables3–5).

Reaction of 2 with KI. A solution of 61.6mg(0.203mmol) of 2 and 90.8mg (0.547mmol) of KI in10ml of dry DMSO containing both biphenyl and 2-butanoneasGCstandardswasstirredfor 2 h in aseptum-cappedvolumetric flask. CG assayon column A gave0.198mmol (98%) of Ph2SO and0.200mmol (99%) ofMeI. Tracesof MeOH andPh2S werealsofound.

Reaction of bromocyclohexane with sodium thiophen-oxide. A solutionof 43.3mg (0.266mmol) of bromocy-clohexaneand91.6mg (0.694mmol) of PhSNain 10mlof DMSOcontainingbiphenylasGCstandardwasstirredfor 1 h. GC assaygave0.172mmol (65%) of cyclohex-ene,0.111mmol (42%)of cyclohexylphenylsulfideand0.0372mmol (11%) of DPDS. PhSH was formed butcouldnotbeassayedbecauseits GCpeakoverlappedthatof thesolvent.ColumnA wasused.

Experiments with sodium alkoxides. (a) NaOMe waspreparedasa dry powderfrom reactionof Na with dryMeOH.A suspensionof 13.8mg(0.256mmol)of NaOMe



in 10ml of MeCNcontainingtolueneasGCstandardwasstirredfor 30min.GConcolumnEfailedtodetectMeOH.PhSH(71mg,0.645mmol) wasinjectedinto thesuspen-sion and stirring was continuedfor 30min. GC againfailed to detectMeOH. Water(0.2ml) wasinjectedandstirringwascontinuedfor 30min.A smallamountof solidremained.GC gave 0.269mmol (105%) of MeOH. Asimilarexperimentwith 10.5mg(0.194mmol)of NaOMeand 1.22g (11.1mmol) of PhSH again gave MeOH(0.185mmol, 95%) only after the addition of 0.2ml ofwater.Thesametypeof experimentswerecarriedoutwithEtSH.With 10.5mg (0.194mmol) of NaOMeand36ml(0.484mmol) of EtSH, MeOH (0.192mmol, 99%) wasobtainedonly after the additionof water.Whena largeramount of EtSH (11.2mmol) was used, MeOH wasdetectableagainonlyaftertheadditionof water,butassaywasthwartedby overlapof theMeOHpeakwith theverylargepeakfrom EtSH.

(b) The sodiumsalt of cyclohexanolwaspreparedbyheatingNa with cyclohexanolunder reflux. The whiteprecipitatewasfiltered,washedwith dry diethyletheranddriedundervacuum.To 20.7mg(0.170mmol)of thesaltwasadded10ml of dry MeCNcontainingnaphthaleneasGC standard.Most of thesaltdissoved.After stirring for30min, GC assayon columnA gave0.162mmol (95%)of cyclohexanol. The experiment was repeatedwithredried(P2O5) MeCNandagaincyclohexanol(98%)wasfoundwithout theneedto addPhSHor water.

Reaction of 1b with PhSH. A solution of 16.2mg(0.039mmol) of 1b and28.3mg (0.257mmol) of PhSHwas madein 3 ml of CD3CN containingnapthaleneasGCstandard.Reaction(decreaseof thepeaksof 1b at8.4and 4.54ppm) had not occurredafter 18h. After thesolutionhadbeenheatedfor 1 h at 100°C, GC showedcyclohexene,ThO,PhSHandDPDS.A solutionof 1b inMeCNgaveThO (105%)andcyclohexene(100%)wheninjectedinto thehot GC inlet.

Acknowledgements

We thank the RobertA. Welch Foundationfor support(GrantD-0028)andProfessorsJ.L. Kice, E. S.Lewis,C.J. Stirling, N. Furukawaand J. Drabowicz for helpfuldiscussions.

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Reactions of nucleophiles with 5-(alkoxy)thianthrenium ions